1. Field of the Invention
The present invention relates to free-space optical laser communication, and more specifically, it relates to techniques for increasing the practical performance and reliability of free-space optical laser communication by resisting channel fading due to atmospheric turbulence and beam pointing errors.
2. Description of Related Art
Free-space optical (FSO) laser communication is becoming an important technology because it allows for a smaller, lighter and more cost-effective communication system at very high data transmission rates. However, one drawback in FSO communication is the strong fading that occurs due to atmospheric scintillation and beam wander. Fading reduces the receiver's signal-to-noise ratio, resulting in data errors. When deployed on a mobile platform such as an aircraft, FSO systems may also suffer from beam pointing errors that temporarily reduce the received power. Transient obstructions may also occur, such as passing birds or clouds. Data collected by the present inventors on 1.3 and 28 km horizontal terrestrial links indicate frequent fades on the order of 10 ms in length. In the presence of such fades, an otherwise excellent communication link can rapidly degrade in terms of the system being a reliable or stable system.
There are many possible optical techniques that may be employed to reduce the effects of fading, and in fact the skilled FSO link designer will combine all of these known techniques in varying degrees:
1. Increase transmitter power. This is not always practical because of eye safety limits, or limitations in the state-of-the-art in transmitter power output capability.
2. Improve the photoreceiver dynamic range, particularly the performance at low input power. Laws of physics dictate that a certain number of photons per bit of information must be received, and some existing photoreceivers are already very near this limit Thus, there will always be fades severe enough to cause errors.
3. Increase the size of the receiver aperture to collect more energy and to improve the average received power through aperture averaging of the scintillated beam. This is not always feasible due to practical matters of physical size and cost
4. Add spatial diversity to the transmitter. Sending multiple optical beams along nominally parallel paths creates spatial de-correlation that tends to average out the effects of scintillation. However, this adds complexity to the transmitter optics, causes great difficulty in time alignment of the beams, and typically requires impractically-large spacing between the beams.
5. Reduce the coherence of the transmitted beam, i.e., use a broadband light source as opposed to a monochromatic laser, to reduce scintillation. Such light sources cannot be focused as effectively, leading to lower average received power.
In addition to these optical techniques, there are also several known techniques involving signal processing, error correction, and network protocols:
1. Retransmit defective data, typically at the packet level. Drawbacks of this method include the requirement for a reverse channel (receiver to transmitter) to request retransmission, added latency, and reduced overall transfer rate because entire packets (consisting of a large number of bits) of data must be retransmitted.
2. Apply forward error correction (FEC) codes that use convolution and interleaving over a block of data [Ref. 1]. This method fails when the fade length exceeds the correction power of the code. It is tempting to propose increasing the convolution length, but that is computationally impractical with very high data speeds (several Gbps) in the face of long fades (10 ms) where hundreds of megabits must be evaluated in each FEC frame. This approach still provides some utility, however, in correcting shorter fades or burst errors.
3. Apply FECs that are coded over multiple wavelengths [Ref. 2]. This method was devised to address the more conventional wideband noise and distortion problems in fiber-optic links and has the same drawbacks as the previous FEC method.
An effective method for addressing long fades is to use delay diversity. [Ref. 3] Multiple copies of each bit or packet of data is transmitted with a suitable delay inserted between each copy. The copies can be transmitted serially on a single wavelength or channel, but this results in a maximum throughput equal to the channel capacity divided by the number of redundant copies. The copies can also be transmitted on different wavelengths or channels [Ref. 4]. Received data on each channel is then temporally adjusted and combined into a single output. That method, as described in the prior art, is fundamentally sound but requires significant improvements in order to become a practical and useful system for FSO link fade mitigation.